This invention relates to heat transfer systems wherein a heat transfer takes place between two bodies of fluid flowing through two conduits that are contiguous to each other. It is typical in such a system for the first body of fluid, which may be water in a domestic hot water system that is heated by the heat transfer, to flow sporadically as, for example, when a tap in the domestic hot water system is opened, and for the second body of fluid to be caused to flow by a pump.